Field of the Invention
The invention lies in the electronics field, with special application in the automotive arts. Specifically, the invention relates to a method for synchronizing computing units connected to one another via a bus system.
In measurement, closed-loop and open-loop control technology, use is frequently made of computer systems provided with a multiplicity of computing units that are connected to one another via a bus system. Particularly in the case of applications in motor vehicle technology, for example in the case of an electro-mechanical braking system, the intention is to realize a control loop via a bus system. For this purpose, in the case of serial data transmission, all connected computing units must have a common time base.
German patent specification DE 41 40 017 C2 discloses a bus system with serial data exchange between computing units that communicate with one another. The computing units each have a dedicated timer system. Time information is exchanged and stored via the data bus. A common time base for the computing units is created by the time values of the local time bases being converted into the common time base in each case with time delays between a data signal delay and its processing being taken into consideration. A receiving computing unit can then convert the transmitted time value from the common time base to its local time base. There is no central clock supply for the individual computing units.
It has been known in the art to realize synchronization by means of a message on the bus. The arrival time of the message enables the receiving computing unit to be synchronized with the transmitter. The temporal resolution of this method is of the order of magnitude of the transmission time of the message. For a CAN bus, for example, this is approximately 200 microseconds at 500 kBit/s.
If a message is unexpectedly delayed, a message with a high priority may also be considerably delayed. This is not acceptable in safety applications in the automotive technology.
A further problem is that all computing units which receive the synchronizing message are synchronized with the transmitter even if the latter outputs an erroneous signal.